I went Frost from the start and i did wonder at the time if i would be drinking constantly but now at level 36 i am just getting the benefit of the Manna saving Talents ( yes i know i went for damage talents first ) i still need to drink every few kills (it also helps to have full Manna against any surprises ) , and that's something i have got used to doing . but it has its benefits in that while drinking i can check out the next mob and see where i need to be which is better than moving into the next attack without checking first.
As you get higher in level there are Manna gems , and the magic 'Evocation spell' to replace manna fast so it does get easier .
Others have put all the main ideas here and they are right , Stay at distance , Use freeze trap , blink or run away and restart attack , if you use instants they will burn your Manna , also with Manna Shield up that will burn Manna if you get hit .
I went for spell Damage gear so have lost a lot of Manna but still have 3000 Manna , and from around level 33 Frost does seem to be the best way for me to go forward.
I have seen Mages levelling in all 3 Talent Trees , they all are viable but of course each to their own destiny , Fire has the lead at lower levels , Frost in mid 30,s and Arcane i have not yet tried out , but i have seen it is still viable ( i still love the graphics on the Arcane missiles ) not sure how it compares against the others though but one day i will try it out.